The present invention relates to a facsimile apparatus capable of transmitting and receiving image data by use of a transmission control procedure based on the G3 (Group III) standard of ITU-T (International Telecommunication Union--Telecommunication Standardization Sector) and a transmission control procedure based on ITU-T Recommendation V.34. More particularly, the present invention is concerned with a facsimile apparatus capable of selecting the data rate of a modem automatically in order to increase the communication rate and reliability.
It is a common practice with a G3 facsimile apparatus to cause a transmitting station to send, at the last stage of Phase B, a training signal of a transmission rate particular to image data to be sent to a receiving station, thereby determining a transmission rate. Specifically, the transmitting and receiving stations try to send image data at the highest possible transmission rate. If the transmission at such a transmission rate fails, the two stations shift down the rate to one allowing image data to be successfully sent. In this manner, it has been customary to only shift down the transmission rate in accordance with the circumstance.
Japanese Patent Laid-Open Publication No. 6-209409 discloses a facsimile apparatus capable of transmitting and receiving image data in ECM (Error Correction Mode). This apparatus sends a PPS-EOM (Pulse Per Second-End Of Message) signal at the end of transmission of one page, returns to Phase B, sets up a transmission rate higher than the rate assigned to the above page, and then executes training check. With this apparatus, it is possible to increase the transmission rate in accordance with the circumstance. However, the problem is that the apparatus does not allow a receiving station to change the transmission rate, and simply relies on the accidental occurrence of an error in changing the transmission rate.
On the other hand, a procedure supplementary to the G3 transmission control procedure, e.g., the procedure prescribed by ITU-T Recommendation V.34, allows a receiving facsimile station to shift down or shift up the transmission rate while the transmission of image data is under way.
Specifically, ITU-T Recommendation V.34 pertains to modems operable at data rates of up to 28,800 bps (bits per second) and applicable to general telephone networks and two-wire point-to-point telephone type exclusive circuits. While Recommendation kV.34 prescribes a modem rate redetermining protocol between pages, how such a protocol is applied is open to choice. Generally, ITU-T ECM is essential when image data are sent by the ITU-T Recommendation V.34 control procedure. With ECM, the transmitting station can automatically resend the error frames included in image data. Specifically, the receiving station informs the transmitting station of the positions of error frames included in received image data, requesting the transmitting station to resend such frames. When errors do not disappear despite repeated resending, the transmitting station executes a shift-down protocol for lowering the data rate and again resends the error frames.
However, in the ITU-T Recommendation V.34 control procedure, the data rate is not determined by the protocol, but determined by a modem sequence at the beginning of communication. Moreover, because a shift-down procedure is executed by a control channel between pages before the start of protocol, a shift-down cannot be determined on the basis of resend request information sent from the receiving station. This obstructs the increase in communication efficiency and the decrease in communication cost. While a shift-down request can be sent not only from the transmitting station but also from the receiving station, a parameter or parameters on which the shift-down request is based are indefinite. As a result, the shift-down is effected arbitrarily.